Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock become especially important and need to be well addressed.
In today's retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, consumers may choose either to adopt a ballast-compatible LLT lamp with an existing ballast used to operate the fluorescent tube or to employ an AC mains-operated LED lamp by removing/bypassing the ballast. Either retrofit application has its advantages and disadvantages. In the former case, although the ballast consumes extra power, it is straightforward to replace the fluorescent tube without rewiring, which consumers have a first impression that it is the best alternative to fluorescent tube lamps. But the fact is that total cost of ownership for this approach is high regardless of very low initial cost. For example, the ballast-compatible LLT lamps work only with particular types of ballasts. If the existing ballast is not compatible with the ballast-compatible LLT lamp, the consumer will have to replace the ballast. Some facilities built long time ago incorporate different types of fixtures, which requires extensive labor for both identifying ballasts and replacing incompatible ones. Moreover, a ballast-compatible LLT lamp can operate longer than the ballast. When an old ballast fails, a new ballast will be needed to replace in order to keep the ballast-compatible LLT lamps working. Maintenance will be complicated, sometimes for lamps and sometimes for ballasts. The incurred cost will preponderate over the initial cost savings by changeover to the ballast-compatible LLT lamps for hundreds of fixtures throughout a facility. When the ballast in a fixture dies, all the ballast-compatible tube lamps in the fixture go out until the ballast is replaced. In addition, replacing a failed ballast requires a certified electrician. The labor costs and messy maintenance costs will be unacceptable to the consumers. From energy saving point of view, a ballast constantly draws power, even when the ballast-compatible LLT lamps are dead or not installed. In this sense, any energy saved while using the ballast-compatible LLT becomes meaningless with the constant energy use by the ballast. Eventually, ballast-compatible LLT lamps are more expensive and less efficient than self-sustaining AC mains-operated LLT lamps.
Even with the above mentioned disadvantages of the ballast-compatible LLT lamps, consumers may still choose to use such lamps, considering only very low initial cost associated with a saving for expensive fixture rewiring. When power is applied to an electronic ballast designed to operate fluorescent tube, a high AC voltage starts to be created to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. When a ballast-compatible LLT lamp used with such a ballast, the high AC voltage originally generated for starting up a fluorescent tube may reach 600 V or even a higher voltage of 950 VAC across a longer lamp. Voltages at these levels represent a strong shock hazard. Person who improperly handles the ballast-compatible LLT lamp by directly touching an exposed bi-pin or electrical connectors can result in severe injury or death. Therefore, a ballast-compatible LLT lamp, as its AC mains-operable counterparts working at 110, 220, or 277 VAC, has a construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps may fail a safety test, which measures through lamp leakage current. Because the high AC voltage from the ballast applies to both opposite ends of the tube when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present a risk of electric shock during re-lamping. Due to this potential shock risk to the person who replaces ballast-compatible LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL) uses its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if the ballast-compatible LLT lamps under test meet the consumer safety requirement. However, this safety issue related to the ballast-compatible LLT lamps has been ignored because such ballast-compatible LLT lamps can pass an initial test in laboratories with a particular electronic ballast. In fields, when used in fixtures that do not have the same brand of the electronic ballast as used in the lab test or have an existing ballast which has been used for years and may be faulty, such lamps may exist an electric shock hazard when used with the ballast. Ironically, a ballast-compatible LLT lamp has compatibility issues with existing ballasts in the fixtures not only to operate the lamp but to show an electric shock hazard. The safety issue needs to be resolved to protect consumers from being injured during relamping.
To verify that there exists such a ballast-dependent electric shock hazard, the inventors have measured leakage current from an exposed bi-pin of a ballast-compatible LLT lamp with the other bi-pin connected to the ballast for various combinations of different brands of electronic ballasts and ballast-compatible LLT lamps with a little different power ratings. In the experiments, three types of brand new and one type of used electronic ballasts and three types of brand new ballast-compatible LLT lamps are used. The results show that all the three lamps used with the used ballast have the largest leakage current among the tests, which could severely burn a person's finger skin although the ballast can normally operate all the three ballast-compatible LLT lamps under test. Other combinations also show unacceptable electric shock levels, burning the tester's finger skin to a certain degree. UL 935 suggests that a measurement instrument with body impedance model and frequency sensitive network be used to measure electric shock current. However, passing maximum meter indicating unit (M.I.U.) of 7.07 required in UL 935 does not mean that there is no shock hazard. As defined in UL 8750, a risk of electric shock exists between any two conductive parts or between a conductive part and earth ground if the open circuit potential is higher than 42.4 V peak AC, and the available current flow between them exceeds 0.5 mA as determined by the leakage current measurement test. In one experiment using a brand new electronic ballast and a new ballast-compatible LLT lamp, an open circuit potential of 75 V rms AC appears at the exposed bi-pin and earth ground, and a current that flow between them reaches 131 mA, well above 0.5 mA limit. The voltage and current at this level represents an unacceptable electric shock hazard to users or installers.
Although electrical power to the entire fixture needs to be disconnected when servicing an existing fluorescent fixture and three or four ballast-compatible LLT lamps in the fixture, it is not always practical in situations where a large number of fixtures are controlled from the same power switch such as in open office areas. In this case, risk of electric shock is unavoidably high to the person who does servicing. Fluorescent lamp ballasts can fail in many failure modes such as leaving and operating burned-out lamps in the fixture, using the wrong size lamps, improper wiring, incorrect line voltage, operating at temperatures below or above the rated limits, power surges, and even the age. However, not all the ballasts fail and stop functioning-many just overheat. So a severe problem occurs when a ballast is still functioning but has significant amount of leakage current that may introduce a shock hazard, and when a user tries to replace a ballast-compatible LLT lamp in the fixture that has such ballast, without knowing the risk of such an electric shock. Many even mistakenly believe that through the electronic ballast as an electrical buffer, there is no risk of electric shock for an exposed bi-pin when the other bi-pin is installed and energized.
When there are various kinds of LLT lamps on the LED lighting market, there are various kinds of configurations of linear fixtures, and misapplications of power supply may occur. For example, installing a ballast-compatible LLT lamp in the fixture that is intended for an application of AC mains of 277 V may burn some of the electronic components not rated at 400 V peak in the lamp, which create a fire hazard. Above all, a power source of AC mains at 277 VAC is different from that of an electronic ballast, which has an internal protection circuitry to shut down the operation of the ballast once short circuit is detected. So the design of a ballast-compatible LLT lamp must take this into account by removing such a risk.